A great variety of handheld power tools have been created by practitioners in the art through the years. These extremely practical and helpful power tools have included circular saws, drills, reciprocating saws, chain saws and the like. A great number of such power tools are electrically powered having integral electric motors which are powered either by battery power sources within the power tool or through electrical connection to a conventional AC power source. Other types of power tools are operated using small gasoline engines. In most, if not all, of such power tools whether electrically or gasoline engine powered, a squeeze type control usually a squeezable trigger is used to control the power tool. In the most common type of arrangement, a movable trigger is supported within or proximate to one of the handle grips used by the operator in manipulating the power tool.
Such power tools provide great advantage in that they are largely or completely portable and thus are often available for use in otherwise difficult situations or environments. In many instances, however, the portability of such power tools and their flexibility of use in challenging environments often leads to substantial safety risks in that the operators of such tools often employ their use in somewhat precarious circumstances such as reaching high overhead or reaching laterally an extending distance to achieve a difficult objective.
In such extended or precarious circumstances, a substantial safety risk arises in that the natural human reflex when slipping or falling or losing balance in such precarious positions leads the operator to squeeze and grip the handle or handles of the power tool harder than usual. In many instances, operators subjected to falling or slipping actually instinctively lock onto the handle including the trigger actuator in a "death grip" type reflex action in which great force is applied to the trigger mechanism. As a result of this tendency or reflex, operators in precarious situations actually increase the risk of injury by such reflex gripping in that the power tool is then thrust into maximum operation adding the danger of a fully energized or fully operating power tool to an already dangerous situation. One can readily see that a risk of falling while itself is potentially dangerous, the potential for serious injury is greatly increased if the operator is holding a tool such as a chain saw, circular saw or reciprocating saw operating at full speed.
Practitioners in the art have recognized a variety of injury potentials within such power tools and have generally attempted to render such power tools safer to operate. For example, U.S. Pat. No. 3,873,796 issued to Worobec, Jr. sets forth a TRIGGER MECHANISM FOR HAND OPERATED POWER DEVICE INCLUDING INDEPENDENTLY OPERABLE LOCKING DEVICES PROVIDING AUTOMATIC LOCK OFF AND MANUAL LOCK ON OPERATION in which a trigger mechanism for hand operated power tool includes a manually operable trigger movable between inoperative and operable positions. A manually operable locking device automatically locks the trigger in its inoperative position and permits the trigger to be manually unlocked for movement between its inoperative and operative positions. A second manually operable locking device permits the trigger to be locked in its operative position.
U.S. Pat. No. 3,854,020 issued to Glover, et al. sets forth a TRIGGER MECHANISM FOR HAND OPERATED POWER DEVICE INCLUDING STATIONARY LOCKING DEVICE WHICH PROVIDES LOCK OFF AND LOCK ON OPERATION in which a trigger mechanism for hand operated power tool includes a manually operable trigger movable between inoperative and operative levels and a stationary locking device which normally locks the trigger in its inoperative level and which permits the trigger to be locked in its operative level.
U.S. Pat. No. 4,006,334 issued to Robotham, et al. sets forth a SAFETY SWITCH FOR POWER TOOL in which a switch and cooperating switch actuating element assure that the switch cannot be moved unless the actuating element is first manipulated through right angle motions in a predetermined sequence. The actuating elements carries a latch which normally holds it in the switch off position. The latch must first be moved laterally against the force of a biasing spring and held to release the actuating element.
U.S. Pat. No. 4,258,368 issued to Arnold, et al. sets forth SAFETY MEANS FOR PREVENTING THE AUTOMATIC RESTART OF A MOTOR in which a control system includes a thyristor switch such as a triac. Upon deenergization of the motor, the thyristor switch switches to its nonconductive state thereby preventing automatic restarting of the motor upon the restoration of power.
U.S. Pat. No. 4,451,865 issued to Warner, et al. sets forth an ELECTRICAL CUT-OUT FOR UNDERVOLTAGE OR POWER LOSS CONDITIONS in which a safety feature within a tool or appliance or other electrically operated device is configured to require that the trigger or on/off switch be returned to the off position before the tool will respond to placing the trigger or on/off position in the on position.
U.S. Pat. No. 3,886,658 issued to Wikoff sets forth a HANDHELD POWER SAW having a safety handle which incorporates a safety switch disposed in a plane transverse to the plane of the power saw's blade. A main handle incorporates an on/off switch and is disposed in the plane of the saw blade. The safety handle comprises a hand grip of generally arcuate configuration that defines an arc of at least ninety degrees and a trigger member which extends along the hand grip's length and is pivotally connected at one end thereto. A switch carried in the hand grip electrically connects the trigger with the saw's power source.
U.S. Pat. No. 4,739,435 issued to Nothofer sets forth a SAFETY DEVICE FOR AN ELECTRIC POWER TOOL having a main switch, electronic controls and a tachometer/generator and a relay. The relay electrically parallels the motor and is activated through the tachometer generator and a safety circuit. Means are provided for switching the motor off at excessively high speeds to avoid overspeed of the power tool.
U.S. Pat. No. 4,794,273 issued to McCullough, et al. sets forth an ON/OFF CONTROL SYSTEM FOR POWER OPERATED HAND TOOLS in which the hand tool is driven by an electric motor, flexible shaft or a source of pressurized air through a supply line. A low power frequency is applied alternately to the hand tool and the supply line while appropriate circuitry detects changes in the frequency of applied signal caused by the operator grasping the hand tool or power supply line.
U.S. Pat. No. 4,839,533 issued to Aga sets forth a REMOTE SAFETY SWITCH in which a foot operated safety switch preferably including an electronic control reset circuit is connected between a source of electrical current and the power cord of an electrically operated device having an on/off switch of its own.
U.S. Pat. No. 4,964,558 issued to Crutcher, et al. sets forth an ELECTROMECHANICAL FASTENER DRIVING TOOL in which a power operated automatic nailing machine or the like includes an elongated handle having a trigger switch and a separate safety switch both controlling the operation of the power nailer.
U.S. Pat. No. 4,970,355 issued to Haeusslein, et al. sets forth a SAFETY DEVICE IN AN ELECTRICALLY POWERED MACHINE ESPECIALLY A HANDHELD POWER TOOL in which a source of electrical energy is controlled by an on/off switch. A switch handle is provided for operating the switch to control delivery of electrical energy to the active mechanism. A stop pawl is provided which prevents the connection of the electrical plug to the machine so long as the on/off switch is in the on position thus preventing an unintentional start up of the tool.
U.S. Pat. No. 5,094,000 issued to Becht, et al. sets forth a HANDHELD POWER TOOL WITH A ROTARY DRIVEN TOOL in which a common housing supports a rotary driven tool and a trigger actuated brake element.
U.S. Pat. No. 5,105,130 issued to Barker, et al. sets forth a KEYBOARD CONTROLLED MULTIFUNCTION POWER TOOL WITH VISUAL DISPLAY in which a multifunction handheld power tool includes an electric motor for driving a tool holder and control means for energizing the motor. A keyboard supported on the housing is used to input a combination of instruction conditions to the control mechanism.
U.S. Pat. No. 5,172,522 issued to Jares sets forth a HANDLE USED ON A HANDHELD GRINDER OR BUFFER in which a generally u-shaped handle is coupled to the head portion of a buffer or grinder to provide additional hand control of the tool.
U.S. Pat. No. 4,617,501 issued to Smith sets forth a CONTROL AND SAFETY SYSTEM FOR ELECTRICALLY POWERED SUBMERSIBLE TOOLS AND LIGHTS in which a control system includes a relay for connecting and disconnecting the external AC power source from the system and a transformer connected to the relay for transforming the AC power to a desired lower voltage together with a converter connected to the transformer for converting the transformed AC to DC power.
U.S. Pat. No. 3,646,298 issued to Weber, et al. sets forth a SWITCH OPERATING TRIGGER WITH INTEGRAL BIASING MEANS AND EXCESSIVE FORCE PROTECTION MEANS in which a grass trimmer having an electric motor, a rotary blade, a handle extending from the motor and a trigger switch on the grip portion of the handle is described. The trigger switch includes an on/off switch for applying power to the motor and a trigger member. Integral with the trigger member is a resilient extension which abuts a fixed surface in the handle to provide a force biasing the trigger in the off direction to provide an improved "feel" for the operator and avoid a "hair trigger characteristic".
While the foregoing described prior art devices have added to the safety of many power tools in operation, they have failed to address the need described above in rendering power tools safer in operation during the reflex locking or gripping circumstances. There remains, therefore, a need in the art for ever safer and more risk free power tool control systems.